Report

Report

Introduction

Bentonite has been used by the taconite industry since the commercialization of the taconite process. Bentonite provides unique binding qualities that promote stable pre fired pellet strength during drying. Laboratory balling tests can be used to determine the balling characteristics of bentonite. Research indicates that one of the most significant detrimental factors affecting the binding qualities of bentonite is the presence of divalent cations (calcium and magnesium) in taconite process water. Both green ball quality and dry ball quality are negatively affected by the presence of divalent cations. This decrease in bentonite effectiveness appears to be due to the ion exchange of sodium in the bentonite (sodium montmorillonite) with divalent cations in the filter cake water. Figure indicates that the zeta potential of bentonite increases as the calcium ion concentration in water increases. For the particular bentonite used for collecting the data illustrated in Fig. the zeta potential of the bentonite became positive when the calcium concentration reached 630-mg/L calcium. Figure also illustrates that the zeta potential of the bentonite remained negative and constant (",-35) when sodium is present in the water rather than calcium. Bentonite zeta-potential tests suggest that as the concentration of calcium in process water increases, the effectiveness of bentonite decreases. Zeta potential tests also suggest that removal of calcium from the filter slurry water prior to balling will improve bentonite binding characteristics, and the addition of sodium to filter slurry water will not affect bentonite binding in green balls (Stone, 1967).

Discussion

Bentonite balling tests with different process water chemistry. Filter cake samples were obtained from two Minnesota taconite facilities, the filter cake sample from each plant was split into three equal samples. These samples were re pulped with water containing I ,OOO-mg/L Ca and then filtered twice with water containing I OO-mg/L Ca and 1 ,OOO-mg/L N a. This produced six filter cake samples. Comparative balling tests were conducted using these six filter cake samples. The average dry compression strength and 18-in.-drop strength for green balls made using water with I ,OOO-ppm sodium were 1.0 kg (2.2Ib) greater and3.1 drops greater, respectively, than for green balls made using water with I OO-ppm calcium. The green ball drop strength and dry ball compression strength that were obtained when green balls were made using water that contained I ,000- ppm calcium were 6.9 drops less and 2.0 kg (4.3 lb) less, respectively, than what was obtained when the water contained I ,OOO-ppm sodium.

Balling tests conducted using magnesium solutions exhibited similar results. The results indicate that approximately SOO-mg/L magnesium is equivalent to 1, OOO-mg/L calcium. One would expect that 600 mg/L magnesium would be equivalent to 1,000 mg/L calcium, based on a concentration calculation using the molecular weight of calcium and magnesium (1,000 mg/L x MW M)MW Ca == 1,000 x 24.3/40 == 60S mg/L). Figures demonstrate the effect that magnesium has on pre fired ball quality. Both magnesium and calcium are divalent cations and have a negative impact on the binding characteristics of ...